Vaults are ubiquitous, highly conserved cellular components found in phylogeny as diverse as mammals, avians, amphibians, the slime mold Dictyostelium discoideum, and the protozoan Trypanosoma brucei. Scanning transmission electron microscopic analysis has shown that the molecular mass of vaults is about 12.9±1 MDa, and cryo-electronmicrograph single particle reconstruction has determined that vaults have an overall dimension of about 420×420×750 Å. Thus, vaults have a greater mass and size than many icosahedral viruses. The function of vaults is currently unknown.
Vaults are ribonucleoprotein particles comprising three different proteins, designated MVP, VPARP and TEP1, and between one and three different untranslated RNA molecules, designated vRNAs. For example, the rat Rattus norvegicus has only one form of vRNA per vault, while humans have three forms of vRNA per vault. The major vault protein, MVP, a 95.8 kDa protein in Rattus norvegicus and a 99.3 kDa protein in humans, is present in 96 copies per vault and accounts for about 75% of the total protein mass of the vault particle. The two other proteins, the vault poly-ADP ribose polymerase, VPARP, a 193.3 kDa protein in humans, and the telomerase/vault associated protein 1, TEP1, a 292 kDa protein in Rattus norvegicus and a 290 kDa protein in humans, are each present in between about 2 and 16 copies per vault.
VPARP, is a poly ADP-ribosyl polymerase apparently unique to vaults. It includes a region of about 350 amino acids that shares 28% identity with the catalytic domain of poly ADP-ribosyl polymerase, PARP, a nuclear protein that catalyzes the formation of ADP-ribose polymers in response to DNA damage. VPARP catalyzes an NAD-dependent poly ADP-ribosylation reaction, and purified vaults have poly ADP-ribosylation activity that targets MVP, as well as VPARP itself.
Cryo-electron microscopy studies have determined that the vaults are hollow, barrel-like structures with two protruding end caps and an invaginated waist. Regular small openings surround the vault cap. These openings are large enough to allow small molecules and ions to enter the interior of the vault. The volume of the internal cavity of the vault is about 5×107 Å3, large enough to enclose two ribosomes.